The world's known oil reserves are dwindling at an ever increasing rate as developing nations industrialize and demand increases. The price of oil exceeded $100 per barrel in 2008 and is very likely to become even more expensive in the future. For electricity generation, there are many alternatives to oil-fired power stations: natural gas, coal, nuclear and hydro-electric power stations are already widely deployed throughout the United States and other industrialized nations. However, burning both natural gas and coal leads to an increase of carbon dioxide levels in our atmosphere and as global warming accelerates and governments seek to address this growing concern, there has been much recent interest in renewable energy sources such as solar, wind and tides.
Most automobiles on the road today use gasoline or diesel fuel that is produced from crude oil. Unless we can convert these vehicles to run on some other form of energy, our transportation options will be significantly impacted by higher oil prices. Some of the cars of the future might run on hydrogen, natural gas, liquefied petroleum gas, bio-diesel or electricity. It is the purpose of this invention to provide a vehicular propulsion option that exploits the various advantages of fuel cells, capacitors and batteries and that optimizes the net energy balance involved in fabricating and operating this power system.
It is well understood that fuel cells are very efficient devices at converting chemical energy into electricity. The most efficient and flexible fuel cell technology available today is the solid oxide fuel cell (SOFC) that can operate on many different kinds of hydrocarbons or on hydrogen gas. The main disadvantages to SOFCs are that they operate at high temperatures and that they emit CO2 if they use hydrocarbon fuel. Other kinds of fuel cells such as direct injection methanol and hydrogen fuel cells run much cooler, though the former still produce CO2 according to the overall chemical reaction:2CH3OH+3O2=2CO2+4H2O
Another drawback to using fuel cells for vehicular applications is that they are very costly per Watt of power generated when compared to a battery or a capacitor. When considering how large a fuel cell is required for a particular vehicular propulsion application, it is most economical to size the fuel cell according to the maximum average power that is required and to provide an auxiliary means of energy storage to provide additional power when accelerating, driving up steep inclines, etc.
Today's hybrid-electric and plug-in electric vehicles mainly use nickel-metal hydride (NiMH) or Li-ion batteries; earlier models that used relatively inexpensive but heavy lead acid batteries have been largely retired. Most pundits believe that in the future, most electric cars will use Li-ion batteries similar to those used in the Tesla Roadster or soon to be released Chevrolet Volt. The battery packs for these cars are very expensive and require a lot of energy to manufacture. Unfortunately, as most users of lithium-ion batteries in cell phones and laptop computers can attest, the capacity and performance of these batteries degrades with age. It remains to be seen how long the latest generation of lithium-ion batteries developed specifically for electric cars will last. The cycle lives and shelf lives measured in the laboratory do not always tally with what happens in actual use due to the many unforeseen circumstances that will be experienced in typical driving conditions.
There is a very real danger that the electric cars we build today may not save the additional energy required for their manufacture. If the battery lifetime is low, the likelihood that the net energy balance will be negative is high. Instead of helping to reduce overall CO2 emissions, electric vehicles made with large, short-lived batteries could actually increase our energy consumption, thereby accelerating global warming. If a battery in an electric car is cycled on average once per day, the battery should have a cycle life far in excess of 5,000 cycles to guarantee that the vehicle will continue to operate for 10 years. Many vehicle owners expect their cars to last significantly longer than this—it is not unusual to see cars that are 30 years old on the roads. The prohibitive cost of replacing a battery in an old electric car will persuade most owners to purchase a new vehicle and the energy required to fabricate an automobile from scratch will be significantly more than that required to fabricate the battery alone. It is therefore desirable that the expensive and critical components used in the vehicular power systems of tomorrow be robust with lifetimes that far exceed what chemical batteries have been able to achieve.
The main reason that electrochemical batteries have limited lifetimes is because their electrodes undergo chemical changes during charging and discharging. These can be in the form of phase changes, structural changes and/or volume changes, all of which can severely degrade the integrity of the electrodes over time and reduce the capacity of the battery. Indeed, the charging and discharging processes in the latest generation lithium-ion batteries must be carefully controlled—overcharging or over-discharging can limit the performance and cause premature failure of the battery.
In contrast, capacitors store their energy as electrical charge on the electrodes. No chemical changes are involved and most capacitors have cycle lives of a million cycles or more, to 100% depth-of-discharge. Capacitors can also be charged and discharged orders of magnitude faster than electrochemical batteries making them particularly attractive for capturing rapidly released energy during regenerative braking. Indeed, many of today's hybrid-electric, plug-in hybrid electric and all-electric vehicles already utilize supercapacitors for this purpose. This provides further proof of the robustness and cycle life of capacitors—brakes often run extremely hot and braking occurs much more frequently than once per day!
Traditional electrostatic and electrolytic capacitors are used widely in electrical circuit applications but can store only relatively small amounts of energy per unit weight or volume. The emergence of electrochemical double layer (EDL) capacitors has now provided a viable alternative to traditional electrochemical batteries where power density and cycle life are more important than energy density. In fact, the latest generation of EDL Supercapacitors have specific energies of ˜25 Wh/kg, approximately the same as lead-acid electrochemical batteries.